An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. An accelerometer may sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device in which the accelerometer is installed can be ascertained. Accelerometers are used in inertial guidance systems, in airbag deployment systems in vehicles, in protection systems for a variety of devices, and many other scientific and engineering systems.
Capacitive-sensing MEMS accelerometer designs are highly desirable for operation in high gravity environments and in miniaturized devices, due to their relatively low cost. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, to vary the output of an energized circuit. One common form of accelerometer is a capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable element or plate that rotates under z-axis acceleration above a substrate. The accelerometer structure can measure at least two distinct capacitances to determine differential or relative capacitance.
Referring to FIGS. 1 and 2, FIG. 1 shows a top view of a prior art capacitive-sensing MEMS sensor 20 constructed as a conventional hinged or “teeter-totter” type accelerometer, and FIG. 2 shows a side view of MEMS sensor 20. MEMS sensor 20 includes a static substrate 22 and a movable element 24 spaced from substrate 22, each of which have opposed planar faces. Substrate 22 has a number of conductive electrode elements 26 of a predetermined configuration deposited on a substrate surface 28 to form capacitor electrodes or “plates.” In an exemplary scenario, electrode elements 26 may operate as excitation or sensing electrodes to receive stimulating signals. Electrode elements 26 may additionally operate as a feedback electrodes when a feedback signal is superimposed on the sensing signal.
Movable element 24, commonly referred to as a “proof mass,” is flexibly suspended above substrate 22 by one or more suspension anchors, or rotational flexures 30, for enabling movable element 24 to pivot or rotate about a rotational axis 32 to form capacitors 34 and 36, labeled C1 and C2, with electrode elements 26. Movable element 24 moves in response to acceleration, thus changing its position relative to the static sensing electrode elements 26. This change in position results in a set of capacitors whose difference, i.e., a differential capacitance, is indicative of acceleration in a direction 37.
When intended for operation as a teeter-totter type accelerometer, a section 38 of movable element 24 on one side of rotational axis 32 is formed with relatively greater mass than a section 40 of movable element 24 on the other side of rotational axis 32. The greater mass of section 38 is typically created by offsetting rotational axis 32. That is, a length 42 between rotational axis 32 and an end 44 of section 38 is greater than a length 46 between rotational axis 32 and an end 48 of section 40. In addition, electrode elements 26 are sized and spaced symmetrically with respect to rotational axis 32 and a longitudinal axis 50 of movable element 24.
The device shown in FIGS. 1 and 2 is a single axis accelerometer which senses acceleration only along the Z axis. However, some applications require the ability to sense acceleration along two or three mutually orthogonal axes. In addition, many MEMS sensor applications require compact size and low cost packaging to meet aggressive cost targets.
Referring now to FIGS. 3 and 4, FIG. 3 shows a top view of a prior art multiple axis MEMS sensor 52, and FIG. 4 shows a side view of multiple axis MEMS sensor 52. MEMS sensor 52 includes a proof mass 54 attached to a number of anchors 56 by a series of springs 58 that are preferably compliant in three mutually orthogonal directions. Anchors 56 are mounted on a die or other substrate 60. Proof mass 54 of MEMS sensor 52 includes X sense fingers 62 and Y sense fingers 64. Each X sense finger 62 is surrounded by two fixed fingers 66 and 68 formed on substrate 60. Likewise, each Y sense finger 64 is surrounded by two fixed fingers 70 and 72 formed on substrate 60. When MEMS sensor 52 experiences acceleration along an X axis 74, the distance between X sense fingers 62 and the adjacent fixed fingers 66 and 68 changes, thus changing the capacitance between these fingers. This change in capacitance is registered by the sense circuitry (not shown) and converted to an output signal representative of the acceleration along X axis 74. Acceleration along a Y axis 76 is sensed in an analogous manner by registering the change in capacitance between Y sense fingers 64 and the corresponding fixed fingers 70 and 72.
Proof mass 54 has opposing sides 78 and 80 which are of unequal mass. This is accomplished by constructing proof mass 54 such that the opposing sides 78 and 80 are essentially equal in thickness and width, but unequal in length. Consequently side 78 has greater mass than side 80, thus causing proof mass 54 to rotate relative to Y axis 76 in response to acceleration along a Z axis 82. This acceleration is sensed by capacitive plates 84 and 86 which are disposed beneath proof mass 54.
The design of MEMS sensor 52 enables a very compact transducer size. In this configuration, XY sensing is coupled with the Z-axis sensing through springs 58. As such, springs 58 need to work as both XY (i.e., linear) springs and Z (i.e., torsional) springs. Unfortunately, it is difficult to optimize the design of springs 58 for both XY (i.e., linear) and Z (i.e., torsional) movement which can result in cross-axis sensing error.
Under acceleration along Z axis 82, the pivot location of proof mass 54 shifts from one end or the other of proof mass 54 since anchors 56 and springs 58 are not centered at a single rotational axis. This “sagging” results in an undesirable second order nonlinearity effect which decreases measurement accuracy and/or increases the complexity of sense circuitry for feedback closed-loop control. Furthermore, the pivot location may change with acceleration frequency so that the common mode and differential mode could have different damping and modal frequency exacerbating the nonlinearity effects.
MEMS sensor applications are calling for lower temperature coefficient of offset (TCO) specifications. The term “offset” refers to the output deviation from its nominal value at the non-excited state of the MEMS sensor. Thus, TCO is a measure of how much thermal stresses effect the performance of a semiconductor device, such as a MEMS sensor. The packaging of MEMS sensor applications often uses materials with dissimilar coefficients of thermal expansion. Thus, an undesirably high TCO can develop during manufacture or operation. These thermal stresses, as well as stresses due to moisture and assembly processes, can result in deformation of the underlying substrate, referred to herein as package stress. The multiple locations of the non-centered anchors 56 on the underlying substrate of MEMS sensor 52 makes it more prone to measurement inaccuracies due to package stress.
Accordingly, what is needed is a compact transducer that can sense along two or more mutually orthogonal axes and that decouples XY sensing from Z sensing to enable optimization of the springs for their corresponding sensing axis and to reduce nonlinearity effects. What is further needed is a compact transducer with reduced sensitivity to package stress.